Kill StatusUpdate, Activity, DmnAnnotation, ContextInfoUpdate, AgentUpdate

Status bar reads directly from Agent and MindState on each render tick.
Activity is now a field on Agent — set by agent code directly, read by
UI via try_lock. DmnAnnotation, ContextInfoUpdate, AgentUpdate were
already dead (no senders).

UiMessage down to 4 variants: TextDelta, Reasoning, Debug, Info.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/mind/mod.rs

@@ -328,7 +328,7 @@ impl Mind {
                 (ag.context.clone(), ag.client_clone())
             };
             let result = learn::score_memories_incremental(
-                &context, max_age as i64, response_window, &client, &ui_tx,
+                &context, max_age as i64, response_window, &client, &ui_tx, &agent,
             ).await;
             {
                 let mut ag = agent.lock().await;